Salt lakes generally refer to lakes having a salinity greater than 50 g·L−1. China has a variety of salt lake resources, including carbonate-type salt lakes predominant in Inner Mongolia, sulfate-type salt lakes predominant in Xinjiang, sulfate-subtype and chloride-type salt lakes predominant in Qaidam Basin in Qinghai, and carbonate-type and sulfate-type salt lakes predominant in Tibet. Salt lakes are rich in many valuable resources, such as potassium, sodium, magnesium, lithium and boron, which are important materials for production of various industrial and agricultural products.
Direct exploitation of salt lakes mainly refer to direct extraction and preliminary processing of various natural resources, such as potassium, magnesium, lithium and sodium, into basic raw materials for chemical industries. In current exploitation of these resources, exploitation of potassium resources has been industrialized and provides an important source of potassium fertilizer and significant economic benefits in China. In brine, trace amount of lithium generally exists together with large amounts of alkali metal and alkali earth metal ions. Because they have similar chemical properties, extraction of lithium therefrom has been facing enormous difficulty, which is compounded by a high level of magnesium that limits lithium extraction from brine (FU Ye and ZHONG Hui, Current research status of separation of salt lake brine having high Mg/Li ratio by precipitation [J], Kuangchan Zonghe Liyong, 2010, 2: 30-32). The current separation process for lithium extraction involves isolation of sodium and potassium first, to leave a bittern having mixed Mg and Li, which are then further separated. By such a process it is very difficult to separate and extract magnesium and lithium because salt lakes having a high Mg/Li ratio are dominant in China. Current main methods for extracting lithium from brine include precipitation, solvent extraction, adsorption, calcination, carbonization, salting out, and the like (HUANG Hao, Studies in techniques for separating magnesium from lithium in acidified aged brine from the West Taijinar Lake in Qinghai [D], Chengdu University of Technology, 2009). Among these methods, ion-exchange adsorption can extract lithium at a yield of 90%, but has a strict requirement for a highly selective adsorbent. Current adsorbents are prepared by a complex method and have a low exchange rate, not suitable for large-scale operation and application. Furthermore, methods like adsorption have strict requirements for process conditions and equipment. Solvent extraction has strict requirements for process conditions, extraction equipment and extracting agents, has a low yield per extraction which is less than 50%, is performed in a complex procedure, causes severe corrosion to equipment, and has a high cost, thereby not applicable for scaled-up production. The salting-out method has serious problems of equipment corrosion and solid entrainment, produces some effects only in laboratories, and has not been well industrialized. The calcination method, although already industrialized, has problems of high energy consumption, incomplete calcination, severe corrosion of equipment, evaporating a large volume of water, and being unsuitable for brine having a high Mg/Li ratio (Jianyuan YANG and Kangming XIA, Process for producing high purity magnesate, lithium carbonate, hydrogen chloride and ammonium chloride [P], CN1724373, 2006). The lithium recovery rate by the calcination method is generally about 80%. Carbonization facilitates large-scale lithium extraction, and has advantages of continuous operation, a low cost, and good product quality, but its development is limited by the gas source of carbon dioxide produced by it (Baocai WANG Lithium-bearing brine resources status and its progress of development technology in China [J], IM&P, 2000, 10: 13-15). The selective semi-permeable membrane method mainly uses an ion-exchange membrane selective for mono-valent ions to cyclically concentrate lithium to obtain lithium-rich, magnesium-lean brine, followed by addition of soda to precipitate and obtain a lithium carbonate product, of which the lithium extraction yield per cycle can reach 80%. However, this method heavily depends on the membrane material, and such materials are proprietary to foreign companies. The precipitation method is a simple, low-cost extraction method, which is mainly suitable for extraction of lithium from salt lake brine having a low Mg/Li ratio. However, most salt lakes in China have a high Mg/Li ratio, and significant presence of magnesium salts will severely affect extraction of lithium and increase the difficulty of lithium extraction by this method, and eventually affect the salt lake lithium industries. Lithium resources extracted by this method are mainly in the form of lithium carbonate, lacking lithium-based functional material products having a high added value. All the methods described above can only extract lithium from brine, while the magnesium left behind by lithium extraction is not developed into a high-performance magnesium-based functional material. Therefore, the isolated magnesium resource is not fully used, and the utilization thereof is low.
Magnesium/aluminum layered double hydroxide (MgAl-LDH) and lithium/aluminum layered double hydroxide (LiAl-LDH) are double metal hydroxides having a layered structure, wherein metal atoms are alternately arranged in layers and anions may be inserted into the inter-layer regions, representing an important class of layered functional materials which find wide applications in the fields of catalysis, adsorption (adsorption of anions or carbon dioxides in solutions), functional aids (flame retardants, UV-blocking agents, and thermal stabilizers), and medicine (DUAN Xue, et al., (eds), Two-dimensional nano composite hydroxides: structure, assembly and function, China Science Publishing & Media Ltd. (Beijing), 2013).